What percentage of Down Syndrome cases are attributed to full trisomy?

Down syndrome, also known as Trisomy 21, can arise from various genetic mechanisms, and the most common form is full trisomy, where there are three copies of chromosome 21 instead of the usual two. This occurs due to nondisjunction during gamete formation, leading to the presence of an extra chromosome in the fertilized egg.

Research indicates that approximately 95% of Down syndrome cases are attributed to this full trisomy. This statistic highlights the predominance of full trisomy as the main cause of Down syndrome, underscoring its significance in genetic counseling and prenatal screening.

Other forms of Down syndrome, such as translocation and mosaicism, account for a smaller percentage of cases. Translocation Down syndrome involves part of chromosome 21 being attached to another chromosome, while mosaic Down syndrome results when there is a mixture of normal and trisomic cells. These alternate forms lead to a lower overall percentage compared to full trisomy, which clearly dominates the prevalence of Down syndrome cases.